Hyperthyroidism in Cats

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Overview of Feline Hyperthyroidism

Feline hyperthyroidism, also referred to as thyrotoxicosis in cats, is one of the most common endocrine disorders in cats. If your adult cat suddenly begins to lose weight despite a voracious appetite, he may have a hormone problem, specifically the hormone produced by the thyroid gland. You should probably get your pet to a veterinarian to be checked for feline hyperthyroidism.

Below is an overview of hyperthyroidism in cats followed by detailed in-depth information on the diagnosis and treatment options for this disease. 

Feline hyperthyroidism has become a widely recognized disorder in cats. It is caused by an unregulated overproduction of thyroid hormone by the thyroid glands, which is usually related to a benign enlargement (growth or tumor) of one or both thyroid lobes. Cancer of the thyroid gland is found in less than 2% of cats. This enlargement of the thyroid gland(s) is referred to as thyroid adenoma or thyroid adenomatous goiter. It is unknown what causes the thyroid to become enlarged.

The thyroid gland consists of two flat lobes shaped like a butterfly and located on either side of the trachea, or windpipe, just below the voice box. These lobes are flattened and cannot be easily palpated. The thyroid gland acts as the thermostat for the metabolic rate of the body, controlling how fast or slow the body functions. Hyperthyroidism can have effects on multiple organ systems, since the increased thyroid hormone levels increase the cat’s metabolic rate.

Hyperthyroidism is diagnosed in cats from age 4 to 20 + years, although this disease is usually diagnosed in older cats (95% are at least 8 years of age). There is no recognized breed or sex predilection for this disease, although purebred cats seem to be less likely to be hyperthyroid.

There is no known cause for hyperthyroidism although canned food and ectoparasiticide exposure has been theorized.

What to Watch For

The classic signs are:

  • Weight loss in spite of an increased appetite
  • Restlessness and/or hyperactivity
  • Increased activity levels or irritability

    Other signs include:

  • Increased thirst
  • Vomiting and or diarrhea
  • Decreased grooming activity
  • Seeking cool areas

    Please note approximately 10% of cats have what is referred to as “apathetic hyperthyroidism” with atypical symptoms. Clinical signs may include lethargy, weight gain and decreased appetite.

  • Diagnosis of Hyperthyroidism in Cats

    Diagnosis can be made by a simple blood test that measures the level of the thyroid hormone (T4).

    Your veterinarian may also perform other diagnostic tests to exclude other diseases, including:

  • Complete medical history and physical examination
  • Complete blood count (CBC) and serum chemistry profile
  • Thoracic radiographs
  • Blood pressure
  • Echocardiogram
  • Electrocardiogram (if an abnormal or irregular heart rate is suspected)
  • A T3 suppression test in hard-to-diagnose cases
  • In some cases a radionuclide scan (used in diagnosis for whole body or individual organ scanning)
  • Treatment of Hyperthyroidism in Cats

    Treatment is directed at controlling excessive secretion of thyroid hormones and can involve a variety of approaches depending on several factors. These include your cat’s overall health, availability of radioactive iodine therapy and cost considerations. There are three main methods of treatment:

  • Radioactive iodine therapy
  • Surgical removal of the abnormal thyroid lobes
  • Medical therapy with Tapazole® (methimazole) and beta-adrenergic blockers (such as atenolol) to reduce some of the symptoms of hyperthyroidism
  • Iodine restricted diet such as Hills y/d
  • Home Care and Prevention

    At home, be sure to administer any medications prescribed by your veterinarian. If your cat is taking Methimazole, a potential adverse effect is loss of appetite, which may be related to liver complications.

    There is no prevention because the cause is not known. However, examination of the thyroid area should be a regular part of any veterinary examination in older cats. If weight loss occurs in your older cat, your veterinarian may recommend a thyroid blood test to screen for this condition.

    In-depth Information on Feline Hyperthyroidism

    Hyperthyroidism is a very common disease in older cats and can produce a variety of symptoms. Although this disease has been reported in cats as young as six years of age, the majority of cases have been in cats over eight years of age. There is no apparent breed or sex tendency.

  • The most common complaints in cases of feline hyperthyroidism are weight loss, polyphagia (increased appetite) and hyperactivity. Other significant signs that have been seen, although reported less frequently, include: polyuria (increased volume of urination), polydipsia (increased thirst), regurgitation, vomiting, diarrhea, increased fecal volume and increased respiratory rate or labored respiration.
  • Increased thyroid hormone levels cause increased energy metabolism and heat production in virtually all body tissues, resulting in increased appetite, weight loss, muscle wasting and increased body temperature.
  • Increased levels of thyroid hormones also interact with the nervous system, causing hyperexcitability, nervousness and muscle tremors.
  • Rapid overeating may cause regurgitation; this is generally seen in multiple-cat households. Vomiting may also result from a direct action of increased thyroid hormone levels on the chemoreceptor trigger zone, a center in the brain that causes vomiting.
  • Intestinal hypermotility associated with hyperthyroidism results in increased frequency of bowel movements and diarrhea.
  • The exact cause of polyuria and polydipsia in the hyperthyroid cat is unknown. Some patients may have concurrent chronic renal failure, which is very common in older cats. Physiologic changes in the kidney, due to increased renal blood flow, may result in kidney inability to concentrate urine normally, resulting in increased production of dilute urine and increased water consumption.
  • Increased levels of circulating thyroid hormones, over a long period, can result in heart changes known as thyrotoxic heart disease. Cats with this condition may exhibit signs of heart failure, with resultant rapid respirations or difficult breathing patterns.
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